1. Field of the Invention
The present invention relates to a device for sorting wafers.
2. Description of the Related Art
Such a sort device is generally known in the prior art. If wafers are being treated batchwise, it is customary to include test wafers in a batch as well as end wafers wherein because they are used as “fill” in the batch, the quality of treatment of these wafers is lower due to the position in the treatment device, and these wafers cannot be used further, or can only be used further after they have been treated. If a batch of wafers comprises, for example, 100 wafers, they are fed in a number of cassettes to the appropriate treatment device, such as a furnace. Normally, such cassettes contain approximately twenty-five wafers, so that in such a case approximately four wafer cassettes are fed to the furnace. A number of these cassettes are filled only with “product” wafers. Moreover, a number of cassettes contain test wafers and the end wafers described above.
In addition to the sorting device, there are normally a separate storage device for cassettes and one or more separate measuring stations for carrying out measurements. This means that in the prior art three or more separate housings are present, each separately provided with a (wafer- or cassette-)handling device and a particle-free environment.
The wafers are handled in the sorting device and in the measuring stations in a particularly particle-free environment. Higher demands are placed on this environment than on a clean room in which wafers are transported in closed pods. From the moment at which the transport pod is opened and the cassette containing wafers is removed therefrom until the moment at which the cassette containing wafers is placed back in the transport pod and the transport pod is closed, this particularly particle-free environment has to be maintained. By providing the pod around the wafer cassette with a standardized door (SMIF, FOUP) and placing the cassette with this door against a wall of the sorting device with a closable opening and simultaneously opening cassette door and wall opening, it is possible to achieve a highly effective separation between clean room and wafer-sorting chamber. As a result, the demands which are imposed on the clean room can be of a low level, with a corresponding saving in the costs, while in the limited volume of the sorting device itself it is possible to maintain an environment which satisfies the most stringent of materials requirements. It is also possible to provide the environment of the sorting device with an inert gas, such as nitrogen. The same applies to the measuring station. There too, the wafers are handled in a particularly particle-free environment. Each sorting station and each measuring station must be provided with a lock mechanism. The storage of wafers in the closed cassettes can take place under less stringent clean-room conditions.
U.S. Pat. No. 4,776,744, from which the preamble of claim 1 is known, discloses a device for sorting wafers. A turn-table is present, having several blades on which an indexer is provided for holding and moving two cassettes. The blades can be moved below a wafer gripper for removal and introduction of wafers out and in the cassettes.